Hello Geraldine,
I'm going to rephrase your questions here just to make sure I understand them correctly.
1. Your Sort-By URLs are using self-referencing rel= canonical tags.
When I looked into this issue I found that some of these pages no longer exist and produce a 404 error page. Example:
https://www.tidy-books.co.uk/childrens-bookcases-shelves/sort-by/position/sort-direction/desc/l/letters:5
I did find some that I was able to access and verified the problem above. You should contact MageWorks to ask if there is a way you can fix this with the SEO plugin you're using. If not, I suggest adding a disallow statement in the robots.txt file for the /sort-by/ folders to keep them from being accessed and indexed. Then I would go into Google Search Console (GSC) and remove that entire directory from the index, as described here: https://support.google.com/webmasters/answer/1663419?hl=en .
2. You have duplicate title tags, mostly caused by the blog and the sort-by URLs discussed above.
If you're not using the Yoast SEO plugin for Wordpress you should be. Without seeing the report and knowing exactly what those URLs are with duplicate title tags, I can't help answer this question very well. Common examples are Tag pages and paginated Category pages. I usually advise blog owners to disallow the Tag pages in the Robots.txt file and removing the directory from Google SERPs using GSC URL Removal Tool, as described above.
3. Image directory pages are causing mobile usability errors in GSC and you have blocked them from being indexed.
Because those index pages do not need to be accessed in order to render the page, I don't see any problem with blocking those in the robots.txt file. However, you do NOT want to block anything that needs to be accessed in order to render the page so I would advise you to use the Robots.txt tester tool in GSC to make sure your Robots.txt code is not inadvertently blocking those image files as well, such as https://www.tidy-books.co.uk/skin/frontend/tidybooks/us/images/eco.png . As long as you aren't blocking access to the actual image files, only to the directory pages, your solution is OK.
It would be better to serve an error code on those pages though. Best practice is to not allow folder index pages to be loaded like that. Otherwise, it creates a bit of a security risk. Often webmasters will show a "403: Forbidden" code on those URLs.
I hope this helps. Please feel free to respond with any updates or clarifications. Hopefully others will chime in now that your issues are broken up into component parts. In the future, you'll get better responses by asking three different questions instead of lumping them all into one. This is because someone my know how to help with one of your questions, but not the others, and are less inclined to answer the one they can help with because they can't answer the others.
Good luck!